The present invention relates generally to communication systems using ultra wideband (UWB) pulses and, more particularly, to techniques for increasing user capacity in UWB transmitters. Communication by UWB pulses, sometimes referred to as impulse radio communication, is a known but not yet widely used technique. The term “impulse radio” is generally used to describe UWB systems in which there is no carrier signal contained within the pulses. The term UWB may also be applied to systems in which the transmitted pulses are bursts of a radio frequency carrier signal. An extremely narrow electromagnetic pulse inherently contains a wide band of frequencies. Adding information to a stream of UWB pulses may be effected by pulse position modulation, wherein the instantaneous value of an information signal sample is used to modulate the position in time of a UWB pulse. At a more fundamental level, UWB pulses may also be modulated by their presence or absence in an otherwise periodic train of pulses. These principles are known in the technical literature and are conveniently summarized in U.S. Pat. No. 5,677,927, issued to Fullerton et al.
Because UWB communication utilizes a large band of the frequency spectrum, it offers the advantages of security and resistance to jamming. Because UWB communication utilizes bandwidth inefficiently, governmental authorization of its use has been limited to relatively low powers. UWB communication, even at low powers, offers the advantages of a relatively long range, the ability to penetrate walls of buildings, and low transceiver cost. However, communication by UWB pulses has some practical limitations, such as the difficulties inherent in applying the technique to multiple users. For example, one possible application of UWB communication systems is for supplying Internet and television connection to homes, as an alternative to coaxial cable, optical fiber cable, or satellite dish communication. UWB communication is ideal for this purpose because it permits the transmission of information at high data rates, using relatively low cost transceivers and processors at user sites. One inherent shortcoming of UWB communication systems, however, is that they allow only one user to receive unique information during any given time interval. The Fullerton et al. patent (U.S. Pat. No. 5,677,927) teaches the use of subcarriers of different frequencies or different waveforms to add channelization to impulse radio signals used in UWB transmission.
An analogous technique for providing multiple user access to communication systems is code division multiple access (CDMA), as used in mobile telephone systems. Multiple users of a CDMA system may share a common frequency band and transmit at the same time, but their information signals are distinguishable from each other because each user is associated with a different code used for spectrum spreading. The codes are said to be orthogonal, which means that they are independently detectable in a receiver. Thus, in CDMA systems, multiple information signal channels may share a single spread-spectrum transmission system. Applying some form of CDMA to UWB transmission would, however, increase the complexity and cost of the modulation and detection equipment.
UWB communication systems that use the presence or absence of a signal to detect whether a user is transmitting during a particular time slot have an inherently low noise signal threshold level. In general if a UWB pulse in a particular time slot has a peak amplitude A, the signal threshold used to determine the presence or absence of a signal is one-half the peak value, or A/2. Thus, when the noise level exceeds an amplitude A/2 the noise will be mistaken for the presence of a signal.
U.S. Pat. No. 6,026,125 discloses techniques for modulating the phase, frequency, bandwidth, amplitude and attenuation of UWB pulses, but it will be appreciated that there is a need for a relatively simple approach for providing better resistance to channel noise in UWB communication systems, thereby facilitating reuse of a time slot by multiple users. The present invention is directed to this end.